I. Field of the Invention
The present invention relates generally to counterweights for piston engines and compressors (hereinafter collectively referred to as xe2x80x9cenginesxe2x80x9d).
II. Description of the Prior Art
The previously known counterweights for engines typically comprise a main body adapted to rotate about a predetermined axis. The main body includes a major lobe on one side of the axis of rotation and a diametrically opposed minor lobe on the opposite side of the axis of rotation. The volume of the major lobe is greater than the volume of the minor lobe to offset the weight of the engine piston.
It is well known, however, that the reduction of volume in the crankcase for the engine effectively reduces engine emissions and increases power output for the engine. Consequently, there have been a number of previously known attempts to construct counterweights which are cylindrical in shape and yet provide the counterweight function.
For example, in one prior art device, plastic inserts are provided on opposite sides of a diametric line bisecting the major and minor lobes of the main body of the counterweight. These inserts are constructed of a lightweight material, such as plastic, so that, with the inserts positioned against the main body of the counterweight, the counterweight assumes a generally cylindrical overall shape. In order to retain the inserts to the main body of the counterweight during rotation of the counterweight, a metal cup is positioned around the counterweight which encases both the main body of the counterweight as well as the inserts. After the inserts are positioned within the metal cup and thus against the main body of the counterweight, the outer edge of the main cup is bent over the outer periphery of both the main body of the counterweight as well as the outer periphery of the inserts thus attaching the inserts to the main body.
While this previously known device has effectively reduced the volume of the crankcase, it has not proven wholly satisfactory for a number of reasons.
One primary disadvantage of this previously known counterweight is that the additional cost of the metal retaining cup for the inserts as well as the manufacturing operation to roll or bend the outer periphery of the retaining cup over the insert unduly increases the overall cost of the crankshaft.
The present invention provides a counterweight which overcomes the above-mentioned disadvantages of the previously known devices.
In brief, the present invention provides a counterweight having a main body which is adapted to rotate about a predetermined axis. The main body includes a major lobe on one side of the axis of rotation and a diametrically opposed minor lobe on the opposite side of the axis of rotation. This minor lobe is adapted for connection with the engine crankpin. Furthermore, the major lobe has a greater volume than the minor lobe and, as such, weighs more than the minor lobe.
The main body further includes two recessed surfaces so that one recessed surface is positioned on each opposite side of a diametric line bisecting the major and minor lobes. Consequently, each recessed surface forms a portion of the outer periphery for both the major and minor lobes.
The counterweight of the present invention further comprises at least one insert which is constructed of a material which is less dense than the material forming the main body for the counterweight. Preferably, the inserts are constructed of plastic while the main body is constructed of metal.
Each insert includes a first surface complementary to at least one of the recessed surfaces on the main body. With the surfaces of the inserts positioned in abutment with the recessed surfaces of the main body, the overall shape of the counterweight is generally cylindrical and solid.
In one embodiment of the invention, two inserts are provided. In order to attach the inserts to the main body, each insert includes a locking tab which protrudes outwardly from its first surface and is received within a like shaped locking recess formed in the recessed surface of the main body. Preferably, the locking tabs are T-shaped in cross section although other shapes may alternatively be used without deviation from the spirit or scope of the invention. Similarly, the locking tabs may alternatively protrude outwardly from the main body and are received in recesses formed in the plastic inserts. The inserts may be press fit onto the main body so that the locking tabs are press fit into the receiving locking recesses. Alternatively, the inserts may be positioned within a generally cylindrical mold and the mold is then filled with a liquid metal. In doing so, the liquid metal encapsulates (or forms) the locking tabs so that, upon solidification of the metal, the metal not only forms the main body for the counterweight but also locks the inserts to the main body.
In yet another preferred embodiment of the present invention, the main body is additionally comprised of a ring and backing, both formed together with the major and minor lobes. Plastic is then formed and inserted into the recesses in the main body.